Individual or team games involving the use of a ball or like object which rebounds from a ball-impacting surface such as a floor, paddle or racket, have rapidly risen in popularity, and many of such games have been elevated from a relatively informal status to the level of national and international competition. Examples of such games are lawn tennis, table tennis and handball. The rules of play governing games of this type have been carefully developed, but few performance standards governing the equipment for such games have been devised.
In the game of handball, for example, the United States Handball Association (U.S.H.A.) rules describe the handball as being of black rubber with a given diameter and weight, but all that is said about ball performance is that the ball must rebound in a non-erratic fashion from 42 to 48 inches when dropped from a height of 70 inches at 68.degree. F. The rules do not specify the energy absorbing characteristics of the surface upon which the ball is dropped, nor do they take into account the variations in symmetry and homogeneity which may give rise to erratic rebounding of the ball from playing surfaces.
The 1974 Rules of Lawn Tennis Play (International Lawn Tennis Federation), as another example, specify that a tennis ball must have a given weight and given dimensions, a particular deformation under load taken 3 axes, and must rebound from 53 to 58 inches when dropped from a height of 100 inches upon a concrete base. Other then the rebound height test, there is no practical test given for insuring the symmetry and homogeneity of the ball so that the ball will not bounce erratically upon a playing surface.
As yet another example, the Rules of United States Table Tennis Association (U.S.T.T.A.) specify that a table tennis ball (a Ping-Pong ball) shall have a given diameter and weight, that it must be spherical and made of celluloid or similar plastic, and that it must rebound within a given range when dropped on to a steel plate. No account taken of the symmetry or homogeneity of the ball. The latter item is of great importance in table tennis because of the spin often imparted to the ball in play and because of the relatively heavy seams between halves of the ball.
The same surprising lack of extrinsic performance standards applies to surfaces upon which games of the above nature are played, and to the rackets or paddles, or the like which are used. When a ball, for example a handball, strikes a playing surface such as the floor, the rebounding of the ball is dependent upon the amount of kinetic energy which the ball loses as it strikes and rebounds from the floor. If the floor is particularly capable of absorbing energy (i.e., if it is made of a deformable material such as soft rubber or the like), the ball will rebound to a lesser degree than it would were the floor made of an extremely, immovable material such as concrete or steel. Similarly, it is evident that a ball (for example, a table tennis ball) will receive different amounts of momentum when struck by a paddle of resilient material such as sponge rubber and when struck by a paddle of unyielding material such as hardwood.
The variances between different brands and grades of balls used in any given sport can often be quite large. Different brands of table tennis balls, for example, have different seams which can greatly effect the performance of the balls. The performance of inflated balls, such as racket balls, tennis balls, basket balls, and the like is greatly dependent upon the internal pressure of the ball. The internal pressure of tennis balls, for example, tends to decrease during storage. The performance of paddles such as those used in table tennis is likely to vary greatly from one brand of paddle to another, and we have found that the type of surface covering (e.g., rubber, sandpaper, etc) which is employed on table tennis paddles is of great importance of stringed rackets, such as tennis rackets, depends greatly upon string tension.
Because of the increasing popularity of games such as those mentioned above, there is a need for some means of standardizing playing equipment, particularly ball and ball rebounding surfaces, so that fairness may be assured between players using different courts or playing surfaces, and different balls. Moreover, some means is needed to measure the actual performance of balls used in such games, and playing surfaces so that the erratic rebounding of balls due to deviations in symmetry and homogeneity can be greatly reduced or eliminated.